1. Field of the Invention
The present invention relates generally to a method of driving a piezoelectric bimorph device used as an actuator and a piezoelectric bimorph device, and more particularly, to a driving method for obtaining a large amount of displacement and a piezoelectric bimorph device suitable for the driving method.
2. Description of the Prior Art
FIGS. 2 and 3 are cross sectional views showing one example of a conventional piezoelectric bimorph device.
A piezoelectric bimorph device 1 shown in FIG. 2 has a structure in which piezoelectric ceramic layers 2 and 3 are laminated while being separated by an electrode 4 and electrodes 5 and 6 are formed on the outer major surfaces thereof. The piezoelectric ceramic layers 2 and 3 are subjected to polarization processing in the directions indicated by arrows as shown.
In the case of driving, a voltage V.sub.0 is applied between the electrodes 5 and 6, as shown in FIG. 2.
On the other hand, in a piezoelectric bimorph device 7 shown in FIG. 3, piezoelectric ceramic layers 8 and 9 are laminated while being separated by an electrode 10, and electrodes 11 and 12 are formed on the outside thereof. The piezoelectric ceramic layers 8 and 9 are subjected to polarization processing in the same direction, as indicated by arrows as shown. In the case of driving, a voltage V.sub.0 is applied between the electrode 10 and the outside electrodes 11 and 12.
The piezoelectric bimorph element 1 shown in FIG. 2 shall be referred to as a piezoelectric bimorph device of a series type because the piezoelectric ceramic layers 2 and 3 are electrically connected in series. On the other hand, the piezoelectric bimorph device 7 shown in FIG. 3 shall be referred to as a piezoelectric bimorph device of a parallel type because the piezoelectric ceramic layers 8 and 9 are electrically connected in parallel.
In obtaining the same amount of displacement, a driving voltage in the piezoelectric bimorph device 7 of a parallel type may be one-half that in the piezoelectric bimorph device 1 of a series type. Consequently, the piezoelectric bimorph device 7 of a parallel type has been conventionally used most commonly.
Meanwhile, in the above described piezoelectric bimorph device, it has been known that when a voltage exceeding the coercive field strength of a piezoelectric body used is applied, the direction of polarization in one of the two piezoelectric ceramic layers is reversed, so that the piezoelectric bimorph device is not bent.
The foregoing will be described more specifically. Let t be the total thickness and let Ec be the coercive field strength in the piezoelectric bimorph devices 1 and 7. In this case, in the piezoelectric bimorph device 1, it has been considered that when a voltage exceeding Ec.times.t is applied, the direction of polarization of the piezoelectric ceramic layer 2 subjected to polarization processing in the opposite direction to the direction in which the voltage is applied is reversed, so that the piezoelectric bimorph device 1 cannot be driven. Consequently, the piezoelectric bimorph device 1 has been conventionally used by applying a voltage of not more than the above described voltage Ec.times.t, so that a large amount of displacement cannot be obtained.
On the other hand, also in the piezoelectric bimorph device 7 shown in FIG. 3, it has been experimentally verified that when a voltage exceeding Ec.times.(t/2) is applied, the direction of polarization is reversed, so that the piezoelectric bimorph device 7 is not driven.